1. Field of the Invention
The invention relates to supply of dry compressed natural gas into fuel tanks. More specifically, the invention provides an apparatus and process that allows complete fast filling of a fuel tank with dry compressed natural gas.
2. Description of the Related Art
The Department of Energy has launched a nationwide program to put 250,000 alternative-fueled vehicles (AFV's) on the road, along with 500 to 1,000 fueling stations in cities of the United States. This new program, known as the "Clean Cities Program", follows on the heels of the recent passage of the Energy Policy Act of 1992 and recommendations of a Federal Fleet Conversion Task Force, established to examine how AFVs can be phased in within the next several years. Natural Gas Fuels, November 1993, p. 9.
While growing concern about the environment, and more specifically about the quality of air, is spurring an interest in the use of compressed natural gas (CNG) as a vehicular fuel, this technology presents several challenges. One of these is the capability to deliver CNG rapidly to the fuel tanks of a user (such as a natural gas vehicle (NGV), from the supply tank of a CNG supplier). According to a recent article, "One of the problems affecting the acceptance of NGV's using fast-fill CNG fueling is the inability to attain a complete tankful. William T. Latto, "Why Can't I Get Full Range When I Fast-Fill My NGV?", Natural Gas Fuels, December 1993, pp. 28-29. The article continues, explaining that, "One of the primary reasons for incomplete fast-fill is the compression heating of the gas in the fuel tank during the fill process." Thus, when natural gas is compressed into a fuel tank, the gas heats up and expands. This effect is particularly acute in warm climates where heat is not readily dissipated from the fuel tank. As a result of the gas expansion, the tank is not quite completely filled. Upon cooling, there is up to about a 25% pressure decrease in the tank as a result of the contraction of cooling gas. Because fuel tanks cannot be completely filled with present fast-fill methods for CNG, the full traveling range of an NGV cannot be realized.
FIG. 1 of the Natural Gas Fuels article illustrates the shortcomings of present CNG fast-fill methods and how an NGV fuel tank cannot be completely filled using the fast-fill methods. As the pressure in the NGV fuel tank increases (as CNG is delivered to the fuel tank) the temperature of the fuel tank also increases. This causes expansion of the gas. When the tank is "full" at a pressure of about 3,000 psi, delivery of CNG into the tank is terminated. However, upon cooling, pressure in the tank decreases and the pressure loss may be as high as 25%. On the other hand, when a time-fill procedure is used, the tank is pressurized much more slowly requiring a very much longer filling time. While this potentially allows complete filling of the fuel tank, the length of time required is inconvenient for a consumer. Thus, while fast-fill procedures solve the fueling time problem by allowing fueling of the tanks in a short time, the fast-fill systems do not allow complete filling of the NGV fuel tank.
According to the Natural Gas Fuels article, one of the presently proposed solutions to solving the "pressure loss problem" is to overpressure the fuel tank during the fast-fill procedure. However, overpressuring a tank poses certain safety hazards and is probably not advisable. Another proposed solution is to cool the incoming gas with a refrigeration system to offset compression heating. This is an expensive proposal and no details of the proposed refrigeration system are supplied. But, it is suggested that a cost analysis is needed to assess the impact of a refrigeration system and high pressure heat exchanger on the cost of providing CNG fast-fill fueling. Another proposed solution is to vaporize liquified natural gas (LNG) with a cold inlet temperature into the fuel tank to offset compression heating. Finally, it is suggested that a heat exchanger should be placed inside the fuel tank to remove heat during the filling operation. This solution appears to be unpractical because a permanent heat exchanger will add weight to the fuel tank and decrease its potential storage space. It is also not practical to have a removable heat exchanger placed inside a high pressure fuel tank.
Aside from these fuel rate and energy conservation aspects, there is a significant corrosion problem. Corrosion fatigue is caused by a combination of corrosive agents found in natural gas--hydrogen sulfide, carbon dioxide, water (or water vapor)--which, together with the pressure cycling associated with the use of CNG equipment, results in the development of stress and corrosion cracks in the equipment. In order to limit these corrosive effects, standards have been proposed for upper limits of water content in CNG for use as a vehicular fuel. These are currently listed as the "draft proposed revisions to SAE J1616," listing upper limits of residual water content in vehicular fuel in 22 urban areas. Of these, the lowest limit is at 5,000 psig set for Milwaukee (0.45 lb/MMSCF), and the highest for Los Angeles and San Diego (3.0 lb/MMSCF); at 3,000 psig, the lowest is 0.5 lb/MMSCF for Milwaukee and the highest is 3.5 lb/MMSCF for Los Angeles and San Diego.
In addition to the corrosion problem, water in CNG could freeze or form hydrates during handling, especially during the desired fast-fill operations. The quantity of water in saturated natural gas at various pressures and temperatures can be estimated from correlations in the literature. Some of these correlations also show a hydrate-formation line indicating that solid hydrates will form when the pressure of natural gas of a specific moisture content is suddenly reduced. For instance, if gas of typical pipeline composition (0.6 gravity) at 2000 psig and 120.degree. F. is expanded to 400 psig, hydrates will form if the gas contains more than 15 lbs. of water per MMSCF. At pressures below about 150 psia, on the other hand, cooling to 32.degree. F. is necessary to precipitate a solid phase, when ordinary ice will form. The hydrates form more readily (i.e., at a higher temperature or lower pressure) with gases of greater density and less readily with very light gases. Thus, for example, at a pressure of 1,000 psia, hydrates form at about 62.degree. F. in natural gas of about 0.60 specific gravity, while they form at about 67.degree. and 71.degree. F., respectively, in gases of 0.75 and 1.00 specific gravity. Thus, it may be expected that compressed natural gas at pressures ranging from about 3,000 to about 5,000 psig, would be highly susceptible to the formation of hydrates if the gas is saturated with water vapor or contains a significant amount of water vapor. Cooling of CNG owing to pressure reduction necessarily takes place during NGV fueling operations, particularly in the case of quick-fill systems.
It is known to remove water from gases by contacting the wet gas with a dehydrating solution that contains a substance that either absorbs or reacts with water. In this process, water vapor is transferred from the gas to the dehydrating solution and dried gas is obtained. Desirably, the dehydrating solution, now containing removed water as a liquid, is regenerated (i.e. dried) and recycled. The regeneration steps typically used include several stages of heating the dehydrating solution to drive off water as water vapor. The substantially water-free dehydrating solution is then recycled for contacting with wet gas.
Typically, the gas industry uses fairly complex gas dryers having very large capacities, ranging up to 75 MMSCFD. These gas dryers include a wet gas dehydrating solution contactor and a dehydrating solution regenerator. U.S. Pat. No. 3,105,748 shows a glycol regenerator utilizing a still column for distilling wet glycol and removing water vapor. It is claimed that the patented stripping section achieves a regenerated glycol purity of 99.95%. Glycol circulation rates vary from about 2 to about 5 gallons of glycol per pound of water to be removed. Thus, to remove 20 lbs. of water per MMSCFD, from about 40 to about 1010 gallons of glycol should be available for each MMSCFD throughput.
What is needed is a method and apparatus that will allow complete filling of a fuel tank with CNG under fast-fill conditions and under climatic conditions that do not allow rapid dissipation of heat or compression. Further, the method and apparatus should reduce or eliminate the problems of icing up and corrosion caused by the presence of water in natural gas.